Consolidated search results and DSL for queries

[jace]

The initial implementation of search as specified in #438 (and #442) left out some advanced implementation details. These are:

1. Autocomplete for search results. Don't wait for the user to submit. Refresh results as they type.
2. Also autocomplete the search query itself, using popular search terms.
3. Consolidated results from across document types, with pills to let the user pick one/many (none selected implies all selected). Replaces the current tabbed layout.
4. DSL for adding filters to search query that invoke fields other than Model.search_vector. Expected fields:
   1. User who created some content: user:username or from:username
   2. User who was tagged in content (pending support for tagging): to:username
   3. Date range: since:yyyy-mm-dd and until: